The first thing the parser does when it performs the
Error
action
is to call a function named
yyerror()
. This happens
before
the parser begins going down the state
stack in search of a state that can handle the
error
symbol.
yyerror()
is
a
lex
and
yacc
library function that simply displays
a text string argument to
stderr
using
fprintf
,
and returns the integer value received from
fprintf
. The
user may choose to supply their own version. See
The lex library routines
for
information about creating a user-defined
yyerror()
.
The simplest
yyerror()
functions either abort the parsing
job or just return so that the parser can perform its standard error
handling.
The
yacc
passes one argument to
yyerror()
: a
character string describing the type of error that just took place.
This string is almost always:
Syntax error
The only other argument strings that might be
passed are:
Not enough space for parser stacks
Parser stack overflow
which
are used when the parser runs out of memory for the state stack.
Once
yyerror()
returns to
yyparse()
, the parser
proceeds popping down the stack in search of a state that can handle
errors.
If another error is encountered soon after the first,
yyerror()
is
not
called again. The parser considers itself
to be in a
potential error
situation until it finds three correct tokens
in a row. This avoids the torrents of error messages that often occur
as the parser wades through input in search of some recognizable sequence.
After the parser has found three correct tokens in a row, it leaves
the potential error situation. If a new error is found later on,
yyerror()
is
called again.
